<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    #result {
      width: 200px;
      height: 100px;
      border: 1px solid
    }
  </style>
  <title>Document</title>
</head>

<body>
  <button>点击发送请求</button>
  <div id="result"></div>

</body>

<script>
  const btn = document.getElementsByTagName('button')[0]
  const result = document.getElementById('result')
  btn.addEventListener('click', function () {
    const xhr = new XMLHttpRequest()
    //超时设置2s 设置
    xhr.timeout = 2000
    //超时回调
    xhr.ontimeout = function () {
      alert('网络异常，请稍后重试！（超时回调）')
    }
    xhr.onerror = function () {
      alert('网络出现问题！就是没有网络的时候！')
    }
    xhr.open('GET', 'http://127.0.0.1:8000/delay')
    xhr.send();
    xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
        if (xhr.status >= 200 && xhr.status < 300) {
          console.log(xhr.response);
          result.innerHTML = xhr.response
        }
      }
    }
  })
</script>

</html>